1. Field
The present disclosure relates generally to tunable oscillator circuits. More particularly, the disclosure relates to a tunable voltage controlled oscillator (VCO) circuit with a relatively constant modulation sensitivity over a wide tuning range.
2. Background
Oscillators are used as stable frequency sources in diverse electronic applications. By way of example, in communication systems, oscillators are often used to provide a stable frequency reference signal for translating information signals to a desired frequency band. In conventional multi-channel communication systems, circuit arrangements employing multiple oscillators may be used to provide a selectable frequency source. This approach is generally used for high speed switching applications. For less critical applications, a more economical approach entails the use of a tunable oscillator circuit comprising a voltage controlled oscillator which can be phased locked to a frequency reference signal.
In conventional VCO designs, varactors (variable capacitors) may be used for frequency tunability. In typical VCO designs, a controlled voltage determines the varactor capacitance which in turn determines the VCO output frequency. The varactors are typically biased around the center of the VCO's frequency tuning range. The frequency tuning characteristics of a typical varactor has two regions: a) a steep capacitance versus voltage slope for certain voltages and b) a saturated capacitance for other voltages. The steep capacitance versus voltage region of the curve may yield increased output phase noise. For example, a small noise voltage at the controlled voltage input causes a relatively large capacitance variation and therefore a noticeable undesired VCO phase noise due to the high sensitivity of the steep curve region. The saturation region of the capacitance versus voltage curve may yield a limited frequency tuning range such that voltages beyond a particular voltage threshold will have very little effect in changing the total capacitance and therefore the VCO output frequency. For example, in one typical design, the frequency tuning range of the VCO is approximately 2 volts (0.4 volts to 2.3 volts), but any voltage higher than 1.6 volts will only minimally change the capacitance and therefore the VCO output frequency.
Accordingly, it would be desirable to have VCO designs which have a wide frequency tuning range but with reduced output phase noise.